As cladding tubes for fuel for nuclear reactors there are normally used thin-walled tubes of zirconium-based alloys, known under the trade name Zircaloy. These alloys contain alloying materials such as thin, iron, nickel, chromium and oxygen. The alloys become irradiation hardened under neutron irradiation. The irradiation hardening results in a brittling of the material and a greatly increased sensitivity to rod damage induced by stress corrosion. To counteract such an occurrence, it is known to provide the cladding tube internally with a layer of zirconium. The suitability of using zirconium for this purpose is due to zirconium having good resistance to hardening when exposed to neutron irradiation and to zirconium in itself being a relatively soft material. A zirconium layer on the inner surface of the cladding tube can therefore be deformed plastically and protect the cladding tubes from stresses occurring in case of power changes during operation.
According to U.S. Pat. No. 4,200,492, there is used in the internal layer zirconium with an impurity content of at least 1000 ppm (parts per million) by weight and at most 5000 ppm. Of the impurities, 200-1200 ppm consist of oxygen, which corresponds to the content of oxygen in commercial, reactor grade zirconium sponge. Contents of other impurities lie within the normal limits for the respective substance in commercial, reactor grade sponge zirconium, which means for aluminum 75 ppm or less, for boron 0.4 ppm or less, for cadmium 0.4 ppm or less, for carbon 270 ppm or less, for chromium 200 ppm or less, for cobalt 20 ppm or less, for copper 50 ppm or less, for hafnium 100 ppm or less, for hydrogen 25 ppm or less, for iron 1500 ppm or less, for magnesium 20 ppm or less, for manganese 50 ppm or less, for molybdenum 50 ppm or less, for nickel 70 ppm or less, for niobium 100 ppm or less, for nitrogen 80 ppm or less, for silicon 120 ppm or less, for tin 50 ppm or less, for tungsten 100 ppm or less, for titanium 50 ppm or less and for uranium 3.5 ppm or less.
According to U.S. Pat. No. 4,372,817, it is known to use in the internal layer zirconium with an impurity content of less than 1000 ppm, preferably less than 500 ppm. Of the impurities, the oxygen content is maintained at a level lower than about 200 ppm.
From U.S. patent application Ser. No. 279,477, filed July 1, 1981, now abandoned, it is known to use in the internal layer zirconium containing 0.1-3 percent by weight molybdenum and/or 0.03-1 percent by weight carbon and/or 0.03-1 percent by weight phosphorus and/or 0.03-1 percent by weight silicon. The zirconium may, in addition, contain the above-mentioned other impurities, included in commercial reactor grade sponge zirconium. The additions of molybdenum, carbon, phosphorus or silicon are assumed in the noted patent application to provide a precipitation of stable phases, such as intermetallic compounds, carbides, phosphides and silicides, in the form of free particles in the zirconium matrix. This precipitation would prevent a grain growth in connection with the manufacture of the tube, so that a structure with smaller grains is obtained in the zirconium than if the additions are not made. The different fine-grained structure would be responsible for the increased resistance to stress corrosion.
To obtain a good resistance to stress corrosion, it has thus previously been assumed that contents of other substances in the zirconium shall be maintained at a very low level or that such additions of other substances shall be made that a precipitation of stable compounds in the form of free particles is achieved in the zirconium matrix.